ORGANOARSENICALS: THE MISSING ARSENIC SINK

Elevated concentrations of arsenic and other uranium (U)-associated elements have been observed in surface and ground waters of the Nueces and San Antonio River watersheds (Texas) that drain the Catahoula formation (a major formation of the Gulf Coast Uranium Province). Intensive U mining from the 1960’s to the 1980’s has resulted in a diverse series of ground water impacts, including artificial groundwater mounds under reclaimed U mines. Surface ponds and wetlands downstream of these mounds are fed by groundwater seeps containing significant concentrations of arsenic.

It is generally assumed that arsenic fate and transport is controlled by the reactivity of the inorganic species and local redox conditions. We hypothesize that production of organoarsenicals by algae in phosphate-limited, surficial waters and subsequent incorporation into organic matter through microbial decomposition is an important missing component of arsenic cycling in these systems. This set of processes may serve as a significant arsenic sequestration mechanism in the surface and ground waters of South Texas and as a natural attenuation mechanism of arsenic contamination.

Contaminated water and sediments were collected from a stock pond and a downstream wetland. Algae obtained from the pond were cultivated under N:P ratios of 20 to 100, conditions which are inclusive of natural environments as well as extremely phosphorus-limited environments. Arsenic concentrations up to 10 mg/L were applied to each set of cultures. After four weeks of growth, the algae were harvested, washed, digested, and analyzed for total arsenic content using stripping voltammetry. Undigested algae samples were extracted using Soxlet extraction and microwave assisted extraction to quantify organoarsenical species. Organoarsenical compounds in sediments were also extracted and quantified using carbohydrate and humic extraction methods and stripping voltammetry.